Tetramethyl biphenyl-2,3,3′,4′-tetracarboxylate

The asymmetric unit of the title compound, C20H18O8, contains two molecules with small geometric differences. The dihedral angles between the benzene rings are 62.94 (12) and 59.99 (12)°. The dihedral angles between the carboxylate groups in the 2- and 3-positions are 81.72 (13) and 65.54 (15)°, respectively. However, the dihedral angles between the carboxylate groups in the 3′ and 4′-positions are 67.24 (15) and 59.98 (17)°, respectively.

The asymmetric unit of the title compound, C 20 H 18 O 8 , contains two molecules with small geometric differences. The dihedral angles between the benzene rings are 62.94 (12) and 59.99 (12) . The dihedral angles between the carboxylate groups in the 2-and 3-positions are 81.72 (13) and 65.54 (15) , respectively. However, the dihedral angles between the carboxylate groups in the 3 0 and 4 0 -positions are 67.24 (15) and 59.98 (17) , respectively.
Experimental 2,3,3',4'-biphenyltetracarboxylic dianhydride (29.4 g, 0.1 mol) and p-toluenesulfonic acid (2.0 g, 0.01 mol) was dissolved in a solution of toluene (100 ml) and methanol (50 ml) in a three-necked flask equipped with a Dean-Stark trap. The mixture was heated to reflux. After 20 h, water and most of the methanol were removed by azeotropic distillation with toluene. The mixture was allowed to cool and was added subsequently 500 ml of H 2 O. The organic phase was washed three times with saturated Na 2 CO 3 and three times with water. Toluene was removed on a rotary evaporator and the residue was recrystallized in EtOH to afford white powder (34.7 g, 90% yield). Single crystals were grown by slow evaporation of a toluene of solution over a period of several days, m.p.382-384 K.

Refinement
H atoms were positioned geometrically (C-H = 0.93-0.96Å) and refined using a riding model with the U iso (H) = 1.2U eq C (for aromatic) and U iso (H) = 1.5U eq C (for methyl). Fig. 1

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F and R-factors based on ALL data will be even larger.